1. Field of the Invention
The present invention relates to an electronic ballast and a lighting fixture using the electronic ballast.
2. Description of Related Art
Generally, an electronic ballast for a discharge lamp comprises a half-bridge inverter, a current transformer, and a load circuit including a discharge lamp. The current transformer includes a detecting winding and a feedback winding. The feedback winding generates a driving signal of switching elements of the half-bridge inverter. Since a core of the current transformer is made of magnetic material, characteristics of the current transformer intends to change according to a heat thereof. Therefore, a current value of the feedback winding changes, so that a switching frequency of the switching elements changes. As a result, an output of the inverter changes, and a lighting output of the discharge lamp changes.
Such an electronic ballast, shown in FIG. 5, is known in Japanese Laid Open Patent Application HEI07-274524 (the ""524 application). The electronic ballast comprises an alternating current power supply (E), a full-wave rectifier 21, a smoothing capacitor C11, an inverter circuit 22 including a current transformer Tr11, and a load circuit including fluorescent lamps FL1, FL2. A first winding Tr12a of the electrical insulating transformer Tr12 is also connected to the current transformer Tr11a. Furthermore, a current detecting circuit 24, arranged between the first winding Tr12a and a capacitor C12, detects a current of the first winding Tr12a corresponding to a current of the fluorescent lamps FL1 and FL2. The current detecting circuit 24 supplies its output current to a base of a transistor Q13 of a current control means 26. The current detecting circuit 24 can control a base current of the transistor Q13. Therefore, the base current of the transistor Q13 changes, so that an impedance of a control winding Tr11d of the current transformer changes to be fix to a designated current of the fluorescent lamps FL1 and FL2.
According to the ""524 application, the current detecting means 24 is only detecting the current of the first winding Tr12a in order to fix the current of the fluorescent lamps FL1 and FL2. The current detecting means 24 can not detect a current of the capacitor C12. Therefore, when the current of the current transformer Tr11 changes due to a heat of the current transformer Tr11, the current detecting means 24 can not properly detect the current of the current transformer Tr11.
Furthermore, another electronic ballast is known in Japanese Patent Registration 3,164,134 (the ""134 patent), in order to avoid a magnetic characteristic change of the current transformer. Such an electronic ballast 50, shown in FIG. 6, comprises an inverter circuit 54 including switching elements Q3, Q4, a current transformer CT4, a magnetic energy control means including a voltage double rectifier circuit 51 and an output controlling circuit 52, and a load circuit 55. A variable resistor of the magnetic energy control means is replaced to an element 53 of a temperature changeable type.
Since a resistance of the element 53 changes due to a heat, a consumption of electricity of the output controlling circuit 52 changes. Therefore, a magnetic energy of the current transformer CT4 changes, so that a saturation interval of the current transformer CT4 also changes. As a result, the switching frequency of the switching elements Q3, Q4 changes to be fix the output of the inverter circuit 54. In case of the ""134 patent, since the resistance of the element 53 changes slowly, the inverter 54 can not quickly response to output.
Furthermore, it is desired that common electronic ballast can operate each different discharge lamp having different lamp characteristics. Generally, the electronic ballast is designed to obtain suitable output of the discharge lamp. In order to design the electronic ballast for one discharge lamp so as to adapt to even the other discharge lamp, the electronic ballast must be designed to generate a rated light output of each discharge lamp. That is, it is advantageous for the electronic ballast to control its output power.
According to one aspect of the invention, an electronic ballast comprises a direct current power supply configured to provide a direct current voltage. A switching circuit, including first and second switching elements, is connected in parallel with the direct current power supply, and is configured to convert the direct current voltage to a high-frequency alternating current. A load circuit, including a discharge lamp, a resonance inductor, and a resonance capacitor, is operated by the high-frequency alternating current. A driving circuit is arranged between the switching circuit and the load circuit. A driving circuit is provided with feedback windings magnetically connected to a detecting winding of the current transformer. A driving circuit is configured to control a switching frequency of the first and second switching elements according to a detected current of the detecting winding. A magnetic energy control means is configured to control a magnetic energy of the current transformer. A current detecting means detects an average current either an output current of the direct current power supply or a current of the switching circuit. A current control means is configured to control the magnetic energy control means, and to fix the average current to a designated value.
According to another aspect of the invention, an electronic ballast comprises a direct current power supply configured to provide a fixed direct current voltage. A switching circuit, including first and second switching elements, is connected in parallel with the direct current power supply, and is configured to convert the direct current voltage to a high-frequency alternating current. A load circuit, including a discharge lamp, a resonance inductor, and a resonance capacitor, is operated by the high-frequency alternating current. A driving circuit is provided with a detecting winding of a current transformer, and is configured to control a switching frequency of the first and second switching elements according to a detected current of the detecting winding. A magnetic energy control means, including a base of a transistor, is configured to control a magnetic energy of the current transformer. A current detecting means detects an average current either an output current of the direct current power supply or a current of the switching circuit. A current control means is configured to control the magnetic energy control means and to fix the average current to a designated value. A current control means is provided with a comparator, wherein the comparator compares a voltage signal of the average current with a reference voltage, and its output supplies to a base current of the base of the transistor.
According to another aspect of the invention, a lighting fixture comprises a body; lamp sockets, and an electronic ballast.
These and other aspects of the invention will be further described in the following drawings and detailed description of the invention.